its a result of our photoreceptors in our eyes becoming desensitized to the stimuli in front of you. this results in the cones to stop responding to change in visual stimulus so when u shift to a blank space, it is still sending the same signal to the brain for a while.

I saw the face, but I can't tell you who it is. My first guess would be JFK? At least optical illusions like this one work for me, unlike the 3D magic eye pictures. I've never been able to get one of those to work. My optometrist tells me that my brain largely ignores signal from my bad eye, and I imagine that makes it very hard for print 3D to work, though for some reason 3D movies still come out fine.